This invention relates to the method of implementing a carrier current communications and control system in large multi-storied buildings.
Carrier current systems utilize existing power distribution systems (systems of wires, raceways, protective and disconnect devices, transformers, etc., installed for the purpose of distributing low frequency (50 to 400 Hertz) alternating current power in a building) to provide the electrical path for non-power related carrier transmissions modulated in some manner in order to transmit voice and/or data individually or in combination. These carrier signals are transmitted throughout the building via the existing power distribution system simultaneously with the transmission of low frequency power. In a carrier current system, transmitters generate modulated carriers which are coupled onto the existing power distribution system via an appropriate coupling network, and receivers receive and demodulate the carrier providing the desired transmission of voice and/or data signals.
At present in order for a carrier current system to effectively transmit and receive information, AC power line coupling devices must be located throughout the building at all transformers, distribution panels and phase separation and protection networks. Each AC power line must be considered a separate path in that at the operating frequencies considered practical for carrier current communications the effects of transformers, separate riser networks, etc., present such high impedance to the communications link that it must be considered an open circuit and therefore a separate line. To unify this plurality of individual links, AC power line coupling devices are used.
AC power line coupling devices are frequency selective networks. When a carrier current system is used to transmit information in a building, it is possible and often desirable to locate transmitters and/or receivers at every location which is either a source or destination of information. In applications where a relatively small number of such locations (either source or destination) are required, individual transmitters and receivers at each location are used. However, where a relatively large number of locations exist, a smaller number of transmitters and receivers is used and some form of multiplex arrangement (time, frequency or a combination thereof) is used to communicate with two or more devices via a single receiver or transmitter. This approach requires the use of a multiplexer with the transmitter and a companion demultiplexer with the receiver.
Moreover, the high attenuation, high and dynamic loading and noise levels normally experienced in the power distribution systems in a building degrade the reliability and performance when communicating over the power lines.
In this environment, the signals of adequate strength transmitted from the transmitting portion at a central station to be received by a large number of remote receivers are liable to be received back on receiving portions at the same central station. Also excessive line loading and low impedance can result from the great many loads of lights and other equipment connected to the distribution network.